1. Field of the Invention
The present invention relates to a motor control method and a motor control apparatus.
2. Description of the Related Art
A motor control apparatus arranged to drive a brushless DC motor typically performs PWM control to supply electric drive currents to the motor. In the PWM control, a plurality of switching elements included in an inverter are turned on and off.
A known motor control apparatus is described, for example, in JP-A 10-248262. In the motor control apparatus described in JP-A 10-248262, three-phase electric drive currents are supplied to a motor by performing PWM control in which three pairs of switching elements included in a three-phase voltage source inverter are turned on and off.
Two types of modulation schemes, a three-phase modulation scheme and a two-phase modulation scheme, are known as modulation schemes used in inverters of such motor control apparatuses. In the three-phase modulation scheme, three pairs of switching elements, each pair being provided for a separate one of three phases of the motor, are turned on and off. Meanwhile, in the two-phase modulation scheme, during every predetermined cycle, one pair of the three pairs of switching elements are fixed in an ON state or an OFF state while the other two pairs of switching elements are turned on and off.
In the case of the two-phase modulation scheme, the number of switchings to be performed is smaller than in the case of the three-phase modulation scheme, and therefore, a power loss due to the switching and an increase in the temperature of the switching elements can be reduced. However, in the case of the two-phase modulation scheme, if the electric drive currents supplied to the motor are small, an effect of a disturbance or the like may become so significant that the waveforms of the electric drive currents may be easily disturbed and an overvoltage and undesirable harmonics may occur.